Current definitions of medical error are too narrow to encompass the lull spectrum of error-related injuries that harm patients, lack precision in differentiating such injuries from adverse outcomes unrelated to errors, and derive largely from hospital studies that lack applicability to ambulatory medicine. This ambiguity about case definitions has yielded misleading epidemiologic data and has excluded from consideration a broad class of errors that cause substantial harm. This study seeks a new perspective on the definition of medical error by gathering the input of primary care patients and their providers. In-depth interviews will be conducted with approximately 45 primary care patients from Virginia and Ohio who are selected to represent important demographic groups. Their perspective is contrasted with that of 10-20 primary care physicians, who will participate in a telephone interview that explores their experience with errors and seeks their reaction to the patients' comments. The interviews focus on defining the types of errors and injuries that patients experience, their relative priority, and the defining characteristics that differentiate them from other adverse events. In addition, physicians are asked to consider the system factors that precipitate or predispose to such errors. The sampling and interview methods are grounded in advanced qualitative research theory and make use of reactor panels composed of other patients and of consultants with expertise in medical err9rs, linguistics, and other social sciences. The study seeks to develop more meaningfi1l patient-oriented typologies of medical error that reflect the injuries that victims of error consider most common and most serious. By incorporating this nomenclature into case definitions, future studies might generate more meaningful epidemiologic data on error incidence and morbidity rates, more appropriately targeted system interventions, and more accurate measures of the effectiveness of patient safety programs.